Integrated: 8375055: C2: Better dead loop detection printout

[Aleksey Shipilev]

On Tue, 13 Jan 2026 08:37:16 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Chasing the maddeningly intermittent CTW failure. When C2 fails dead loop verification checks, it prints:
> 
> 
> # Internal Error (/home/shipilev/shipilev-jdk/src/hotspot/share/opto/phaseX.cpp:784), pid=64465, tid=917
> # assert(no_dead_loop) failed: dead loop detected
> 
> 
> It also dumps the bad node graph to tty. This is not really convenient in automated testing and/or driver tests like CTW. With this fix, we now print:
> 
> 
> #  Internal Error (/home/shade/trunks/jdk/src/hotspot/share/opto/phaseX.cpp:776), pid=973305, tid=973372
> #  fatal error: Dead loop detected, node references itself: CastPP
> 
> 
> ...which allows to have more clues where things may go wrong, and allows to classify the failures better as well.
> 
> Additional testing:
>  - [x] Ad-hoc crashes with selected seeds
>  - [x] Linux x86_64 server fastdebug, `hotspot_compiler`
>  - [x] Linux x86_64 server fastdebug, `applications/ctw/modules`

This pull request has now been integrated.

Changeset: 1b6c2bdd
Author:    Aleksey Shipilev <shade at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/1b6c2bdd7b57891ed35e3c067871d2c0bf282824
Stats:     30 lines in 1 file changed: 12 ins; 2 del; 16 mod

8375055: C2: Better dead loop detection printout

Reviewed-by: chagedorn, qamai

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PR: https://git.openjdk.org/jdk/pull/29185